1. Setting command-line make variable NOASSERT=1 adds -DNDEBUG and builds in a separate directory. By default, we still get Release+Asserts.
2. Add "(void)foo;" as necessary when foo is only used in an assert(), to remove warnings.
3. Minimize inclusion of llvm/Support/Timer.h because it adds warnings.
4. Call validateLiveness() only when asserts are enabled, because it's relatively expensive.
BUG= none
R=jvoung@chromium.org
Review URL: https://codereview.chromium.org/623493002
